Final answer:
Iodine is necessary for the normal production of thyroxine, a thyroid hormone important for controlling the body's metabolic rate. A deficiency in iodine can lead to hypothyroidism and goiter.
Step-by-step explanation:
To produce thyroxine normally, iodine is essential. Thyroxine (T4) and triiodothyronine (T3) are thyroid hormones that control the body's basal metabolic rate and regulate various physiological functions. The production of these hormones is contingent upon the availability of iodine for the synthesis of thyroxine within the thyroid gland.
In the absence of iodine, the synthesis of T3 and T4 is impeded, potentially leading to hypothyroidism and the development of a goiter. Thus, the correct answer to which substance is necessary for thyroxine to be produced normally is A. Iodine.
Calcitonin, another hormone produced by the thyroid, helps regulate blood calcium levels but is not directly involved in the production of thyroxine. Magnesium, phosphorus, calcium, and vitamin D are also important nutrients, but they do not play a direct role in the synthesis of thyroid hormones.